A wormhole is a "tunnel" connecting two separate points in space-time. It consists of two apertures in space-time connected by a conduit outside normal space, through subspace. Since this conduit is shorter than the distance between the two points in normal space, it allows rapid travel between the two points.

If a wormhole was large and stable enough, a starship (or other traveler) could travel through it. A wormhole could also connect two different points in time as well. As wormholes collapse, they become smaller and are known as micro-wormholes. (VOY: "Eye of the Needle")

The first test of this technique produced a wormhole that was stable for 23.4 seconds. On the second test, a class-4 probe was launched through the wormhole to simulate the passage of an interstellar spacecraft. However, the probe's shields unexpectedly interacted with the tetryon field, collapsing the wormhole and producing a massive gravitonshock wave that heavily damaged the Defiant. (DS9: "Rejoined")

The MIDAS array was able to create an artificial micro-wormhole that connected the Alpha Quadrant to the Delta Quadrant, but it was only stable for a brief moment. (VOY: "Pathfinder")

The apparent dissimilarity between the wormhole featured in Star Trek: The Motion Picture and other wormholes featured in subsequent episodes was somewhat explained in VOY: "Counterpoint" as Torat claims that the term "wormhole" is a layman's term that covers any number of phenomena.